Bi-directional network ring topologies are gaining in popularity, particularly in Internet Protocol (IP) networks. Such networks provide efficient bandwidth utilization by enabling data to be transferred between any pair of nodes in either direction around the ring, while maintaining fast protection against faults. The two opposing traffic directions are commonly referred to as an inner ringlet and an outer ringlet, or ringlet 0 and ringlet 1. It will be understood, however, that in the context of the present patent application and in the claims, the terms “inner” and “outer,” as well as other terms such as “east” and “west” or “right” and “left,” are used arbitrarily to distinguish between the two opposing directions of packet flow in a ring network. These terms are chosen solely for convenience of explanation, and do not necessarily bear any relation to the physical characteristics of the network.
The leading bi-directional protocol for high-speed packet rings is the Resilient Packet Ring (RPR) protocol, which has been approved as IEEE standard 802.17, “Part 17: Resilient Packet Ring (RPR) Access Method & Physical Layer Specifications,” which is incorporated herein by reference. Using the RPR protocol, each node (commonly referred to as a “station”) in a ring network has a RPR Medium Access Control (MAC) address and can communicate directly with all other nodes through either ringlet. Each packet sent over either of the ringlets carries a header indicating its RPR MAC destination address. The receiving node recognizes its address in the header and strips the packet from the ring. All other nodes pass the packet onward transparently around the ring.
Nodes in a RPR network use a topology discovery mechanism (described in Chapter 10 and Annex K of the standard) to automatically keep track of the topology of the ring. Topology messages are broadcast from each node to the other nodes on the ring. Each node constructs a topology map, containing information about the location, capabilities, and “health” of other nodes on the ring. Topology messages are generated periodically and upon the detection of changes in local status. When a node is removed or a fiber span between nodes fails, the nodes adjacent to the failure record the status in their topology maps and send protection messages around the ring. All the nodes update their topology maps to reflect the change in connectivity.
The RPR standard (Annex E) also defines a mechanism for bridging between 802.1D and 802.1Q LANs via the ring network. Bridging of this sort is carried out by bridge nodes on the ring, which connect the ring to other LANs. When a bridge node receives a packet from another LAN, it adds a RPR header with an appropriate RPR MAC destination address and forwards the packet across the ring. If the particular RPR MAC address for the packet is unknown, the bridge node uses a broadcast MAC address to flood the packet to all the nodes on the ring.
Busi et al. describe methods for making transparent local area network (LAN) connections over a RPR network in U.S. Patent Application Publications US 2003/0074469 A1 and US 2004/0022268 A1, whose disclosures are incorporated herein by reference. A transparent LAN service (better known as a Virtual Private LAN service—VPLS) provides bridge-like functionality between multiple sites over a large network.
General methods for creating a VPLS, not specifically related to the RPR context, are described by Kompella et al., in “Virtual Private LAN Service” (May, 2004) and by Lasserre et al., in “Virtual Private LAN Services over MPLS” (April, 2004), which are incorporated herein by reference. Users connect to the VPLS via regular Ethernet interfaces. The VPLS entity itself is formed by virtual connections (referred to as “Pseudo-Wires,” or PWs) between the nodes to which the users are connected.
Every node in a VPLS acts as a virtual bridge. A virtual bridge node has “virtual ports,” which are the endpoints of PWs that are part of the VPLS. The interfaces to which the users are actually connected are physical ports at the network edges. Both virtual and real interfaces are treated identically from the point of view of frame forwarding and MAC address learning. A single provider node can participate in multiple VPLS instances, each belonging to different users. From the perspective of the end-user, the VPLS network is transparent. The user is provided with the illusion that the provider network is a single LAN domain. User nodes on different physical LANs can thus be joined together through VPLS connections to define a virtual private network (VPN), which appears to the users to be a single Ethernet LAN.